When sailing do you put.......

[temptress]

I have an Autoprop and a Yanmar 4JH-2E. I have the same issue. I need to engague forward gear to lock the prop. (in reverse it keeps spinning) and when traveling at speed I need to start in gear. This has never been a problem and I checked with yanmar and buntons when I first fitted the prop who said - no problem.

As to weather a fixed prop retes less drag when spinning or fixed well that's a whole other subject.

Before I fitted the autoprop I did some testsover a series of races with a fixed 3 blade prop.

Through expeimentation I found that:

When my boatspeed was below 5 Knots letting the prop spin took at least 0.5 and sometimes as much as 1.1 knots off my speed through the water.

When my boatspeed was between 5 and 8 knots I lost about 0.4 knots to a spinning prop.

Over 8 knots and I could not acurately record the difrance it made.

My conclusion was that letting the fixed prop spin slowed me down and I did not do it. I observed a proportional correlation bettween the loss of speed from the spinnig prop and boat speed which became harder to measure the faster I went.

At this point I bought an Autoprop and there is no dbout that I sail faster with the prop fixed.

 

[geoffatstanpit]

The 100 rpm figure is probably the key to this argument.
My tests (reported at regular intervals on this forum!) gave 4.7Kg drag on a Merc 5hp "sailing" prop in neutral (spinning) and 11.7 Kg with it locked (in reverse) - both at 5 knots. I also measured the static friction of my prop in neutral against a typical-looking inboard prop (on a Catalac that happened to be ashore nearby) and the friction was 22 times greater on the inboard. Inboard props probably rotate at about 100 rpm (guess!) so probably everyone's right. If the spinning prop has to do work to spin it will take drag x speed in power to do it.

 

[Sailfree]

With a fixed prop, Yanmar on modern gearboxes state do not stop prop rotating by engaging astern as it can overstress the gearbox.

The fact is that a boat uses less energy and hence goes faster with prop stationary. It also causes less wear.

With a folding or feathering prop you can engage astern to stop prop rotating.

This is often further complicated by boat builders manuals contridicting the engine manufacturers and stating "engage astern when sailing"!

In my case I changed to a Variprop feathering propeller as I could not stand the noise/thought of the prop continuously rotating!

 

[Largslout]

Thanks for that Sailfree and thanks to everyone for info however I think I may be more confused now!!! So what would you lot do with the Yanmar shaft drive and fixed 3 blades? Can leaving it in astern really overstress the box?

 

[Woodlouse]

For another example, when the engine fails on a helicopter the pilot lets the rotor blades spin freely as it caused far more drag than a fixed blade. Also look at sycamor seeds. If they didn't spin then they'd drop like rocks.

 

[steve_cronin]

Stand by for a LOT of flak!!!

When I took your viewpoint some months ago, I was shouted down by all and sundry, some of whom ought to have known better.

Steve cronin

 

[graham]

[ QUOTE ]
For another example, when the engine fails on a helicopter the pilot lets the rotor blades spin freely as it caused far more drag than a fixed blade. Also look at sycamor seeds. If they didn't spin then they'd drop like rocks.

[/ QUOTE ]

not a pilot but been a passenger on quite a few.A pilot told me that in an engine failure the rotors are allowed to freewheel on the decent so they can reverse the angle of them immmediately before the crash to provide a small amount of lift from the momentum.This was very comforting to know /forums/images/graemlins/wink.gif

 

[Sailfree]

I have a 75hp Yanmar with shaft and 3 bladed fixed prop originally. I took this up with Yanmar technical dept, a couple of Yanmar dealers and Seaventures that supplied the boat with the conflicting advice in the manuals!

I had the same problem that gearbox stuck solidly in astern and I had to start engine in astern and quickly change to neutral. Interestingly if it had been a USA spec boat it would never have started as they have a safety cut out preventing starting engines in gear!

I came to the conclusion there was a serious risk of damage as evidence by the inability to move the gear lever. I hated hearing the prop turning and the unecessary wear so as I said I fitted a Variprop feathering prop. Great piece of kit and no problems with 2 charter seasons some 650hrs of engine.

 

[charles_reed]

I just leave my 3YM20 in Forward gear, when under way.

This has more to do with locking the Brunton Autoprop, than anything else - if I don't the prop starts freewheeling at about 3kts.

Though it's more difficult to get out of gear when the engine is running it certainly isn't impossible.

Having said that one usually needs forward gear on starting the engine so why the problem?

 

[charles_reed]

In practice

Bruntons specifically recommend Forward NOT Reverse. Most would categorise theirs as a feathering prop.

Reverse is used for locking fixed props, more usually.

 

[charles_reed]

Why not get in touch with A E Barrus

rather than rely upon the collective wisdom of this forum

For 3 years I locked my 3-blade fixed prop using reverse (it used to be able to windmill quite easily in forward) and it was the forward cone-clutch actuator which was worn, when I stripped it.

With a cone-clutch box like the yanmar I can't see any reason why you'd "strain" the box.

A very different matter to leaving a hydraulic box in gear - definite knackeration.

 

[charles_reed]

My experience..

corroborates your comments - the available published evidence on whether props need to be fixed or not tends to suggest that at low speeds the drag produced by a spinning prop is a greater part of the total drag factor than at higher speeds when wave-making becomes the major component.

 

[doris]

Had exacly this prob on a chartered Benny Oceanis 323 down in Turkey a couple of weeks ago. Same engine I think. Suppliers said, sure, stick it in reverse. Had to start the motor and immediately put it in neutral.

 

[charles_reed]

The reason

you had the variability would probably be selecting forward gear after the engine stopped turning.

The cone clutches need energising by the drive-shaft rotation to lock.

 

[Danny Jo]

Re: The reason

[ QUOTE ]
The cone clutches need energising by the drive-shaft rotation to lock.

[/ QUOTE ] Many thanks. This solves a really niggly puzzle.

Is it safe to stop the engine while still in gear? I have always selected neutral first.

 

[bilbobaggins]

[ QUOTE ]
reverse the angle of them immediately before the crash to provide a small amount of lift from the momentum.

[/ QUOTE ]

This 'pulling in collective pitch' is one recommended procedure following power failure in heli-choppers. Another such recommended procedure involves as many 'Our Father's and 'Hail Mary's' as you can get in.....

Choosing exactly the right moment for this 'pulling all the collective' is hugely difficult, and the results of doing it a bit too early, or a bit too late, are exactly the same. The heli-chopper hits the ground very hard indeed....

This is, for obvious reasons, an kind-of expensive skill to practice in the wild, so it is not much done.

Hence the little notices that read 'In the event of ditching or other involuntary and uncontrolled landing, place your head firmly between your knees and kiss your ass goodbye!'

/forums/images/graemlins/wink.gif

Oh, and in case you're wondering, the only place that's safe to sit, in the event of a helicopter crash landing, is in the next county....

/forums/images/graemlins/wink.gif

 

[duncanmack]

Dear Phil

Quote
"On the point of gearbox lubrication. Motorcycles, cars and commercials use splash lubrication as the sole method. I haven't had a marine gearbox to bits, are they different?"

Most commercials have an oil pump in the gearbox, driven off the input side of the box.

Splash feed for the other boxes. But it is dependant on all the innards whirling round to create the splash. In neutral this doesn't happen.

SOP for recovery of most RWD cars (and all CVs) . Remove propshaft if vehicle has to be flat towed or front-end suspended.
If this is not possible then have someone in the cab of the casualty and get him/her to put the gearbox into gear from time to time but NOT to engage the clutch ( for obvious reasons) as this will throw oil round the box and prevent damage to the internals.

Once upon a time I/we ignored this advice and it cost me a gearbox. Salutary lesson. Tho it was a 300 mile recovery....

 

[AngusMcDoon]

There have been a few helicopter autorotation myths on this thread. I know it's not to do with the original subject, but it's similar to props freewheeling!

I'm a heli pilot, so here's what happens if an engine failure occurs in powered flight on a chopper...

Within a second or two of engine failure the low rotor speed alarm will go off. Helicopter rotors run at near constant speed in normal flight. As power has failed the rotor speed will drop rapidly (assuming normal powered flight), and hence the alarm will go off.

The pilot's trained response is to lower the collective lever immediately, flare (nose up) and remove tail rotor pitch. This swaps the pitch on the main blades as the air now going upwards through the disc is driving the blades rather than the blades driving the air (see note). The intention is to get the rotor speed back up to 100%. If the rotor speed drops much below 80% the blades will stall and rotor speed will never recover. Death will follow soon after, so lowering that lever is pretty important. In the lightest helis like a R22 the pilot has about 1 second to get the lever down.

The heli should now be established in autorotation, which is a rapid but controlled descent. The heli can be steered and forward speed controlled within limits. A landing site is chosen and radio stuff is done.

If autorotation is established and the pilot did nothing other than steer into clear ground the passengers would survive. The heli wouldn't, because the descent rate is about 1500 feet per minute, but that would be a survivable crash.

However, the intention is to save the heli as well. For this the pilot does a flare at about 25 feet. This slows down forward speed (50 to 60 knots in the autorotation), slows down rate of descent, and speeds up the rotors to get some momentum in them. At about 10 feet the pilot levels out by bringing the nose forward, and then raises the collective lever. This uses the momentum in the blades (very little in a light heli) to cushion the landing for touchdown. There will usually still be some forward speed, maybe 10 to 20 knots, so a sliding run-on landing is done. As the heli slows rapidly sliding on its skids or wheels, the tail rotor is used to keep the nose travelling in the direction of slide.

If well done, no damage is done to the heli at all.


Now some myth busting...

[ QUOTE ]
This 'pulling in collective pitch' is one recommended procedure following power failure in heli-choppers.

[/ QUOTE ]

Nope, the opposite is done. When power fails, collective is lowered. Pull in pitch and the blades will stall irrecoverably instantly, and a fatal crash will follow. Pitch is pulled back in at 5-10 feet above ground, long after the engine has failed. There is no other recommended procedure. Anything else or doing nothing after an engine failure will result in death.

[ QUOTE ]
Choosing exactly the right moment for this 'pulling all the collective' is hugely difficult

[/ QUOTE ]

The difficult part is judging when to flare at 25 feet above ground, it's a lot later than your brain wants to do it. Get that right though and the 'pulling all the collective' when the nose has been levelled is the easy bit, before the next hard bit, the run on landing.

[ QUOTE ]
results of doing it a bit too early, or a bit too late, are exactly the same. The heli-chopper hits the ground very hard indeed....


[/ QUOTE ]

If the autorotation has been established, the heli will hit the ground hard, but survivably, if nothing is done at the bottom at all. This is what the instructors say, I have never tried hitting the ground hard though.

[ QUOTE ]
This is, for obvious reasons, an kind-of expensive skill to practice in the wild, so it is not much done.


[/ QUOTE ]

Simulated Engine Off Landings are perhaps the most practiced safety procedure heli pilots do in their training and subsequent refresher training. The autorotation practices are done with no engine power down to the last moment after the flare raising the lever to cushion the landing, where the instructor will pull engine power back in about a second or two before a touchdown would occurr. Autorotations all the way down to the ground will be demonstrated by instructors, or practiced with the instructor also having hands on the controls. The reason why ordinary pilots don't take them all the way down to the ground unpowered is because in the light training helis it is easy to roll them during the run-on landing sliding part - a survivable but expensive outcome.

There's a video of a practive autorotation in a 2-seat trainer here...

Practice auto

You can hear the instructor cut the engine, see the autorotation descent, the flare, the level off, and then you can hear the engine come back in just before what would be the run on landing.

When leaning to fly a heli the student does this again, and again, and again, but they never get boring!

Back to the point of more or less drag with a fixed propellor, I don't know. Just because a helicopter gets high drag by having a rotating blade I don't think can be applied to boat props. They are a very different shape, and the heli rotors have pitch control. Also an autorotating heli blade has a driving, a driven and a stalled region of the disc, which a boat prop won't have.

Note: It's actually a bit more complicated than this with a driven, stalled and driving region of the disc, but that's a reasonable simplification.

 

[AngusMcDoon]

Re: Stand by for a LOT of flak!!!

I've had to learn the theory as to why autorotations work for helicopters as part of my pilot training. I'm not sure if it is applicable to freewheeling boat props - see what you think.

The lift and drag any bit of an aerofoil generates depends on these 3 things (and other less important factors like fluid density)...

1) its cross section shape - fixed in a heli blade

2) its speed through the air - lower near the hub, faster near the tip

3) the angle of attack to the incoming airflow. This varies along a heli blade because of the ratio of the speed of the blade through the air in horizontal and vertical directions, and because the blades have a slight twist. The blade tip is moving much faster horizontally that the blade root, yet the whole blade feels the same vertical speed through the air all the way along the blade. Therefore this ratio changes along the blade, and hence the angle of attack of the incoming air changes along the blade.

Number 3 is important in autorotations. Because the angle of attack changes dramatically along the blade the lift conditions vary completely from root to tip. What happens is that there are 3 different regions - the driving region, the stalled region and the driven region. The upcoming air drives the blades in one region, because of its angle of attack. This keeps the blades rotating. However, this rotation at the tips where the angle of attack is very different now causes air to be driven downwards - what the engine normally does. So it's the driving region that keeps the blades turning, and the driven region that creates the drag.

Does all this apply to freewheeling props? I would expect them to have a driving, stalled and driven region too. How much drag does this create? Difficult to tell, it depends very much on the geometry of the blades and the rotation and forward speeds.

Heli blades and hubs are very carefully designed to be able to autorotate, which they must to get an approval to fly. Marine props are not designed to autorotate in this manner creating significant drag from a driven region, but they just might by chance.

So does more drag result from a freewheeling or locked prop? I reckon sometimes it might, but mostly probably won't. And that I reckon is why there are so many contradictory views about.